Apparatus for detecting arcing and overcurrents in dc electrical systems subject to cyclic disturbances

ABSTRACT

Arcing in dc electrical systems subject to cyclic disturbances is detected by a current sensor, and a bandpass filter sensitive to arcing noise, which filters the sensor current. The absolute value of the bandpass filtered signal is integrated over repetitive intervals in a resettable analog integrator. A low cost microprocessor samples and resets the analog integrator at a frequency below the pass band and generates a time attenuated accumulation of the samples that produces an arcing signal if it reaches a predetermined limit. To improve nuisance immunity, the sensed dc current is compared in a high speed comparator to the average sensed dc current, and only samples taken by the microprocessor during a predetermined time period after the sensed dc current exceeds the average sensed dc current in a predetermined manner are accumulated. The average sensed dc current is also used by the microprocessor to provide overload protection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus and a method for detecting arcing indc electrical systems and especially those subject to cyclicdisturbances such ac ripple and pulse width modulated loads.

2. Background Information

It is common to provide overload, and sometimes overcurrent, protectionin dc electrical systems. Overload protection is typically provided byeither a thermal element which emulates the heating of the distributionwiring and opens a contact when the bimetal reaches a certaintemperature, or an electronic circuit which simulates the same thermalprocess. Overcurrent protection is typically provided by aninstantaneous trip feature which opens the circuit breaker rapidly ifthe current exceeds a particular threshold, such as would be reached bya short circuit, and is implemented by a magnetic trip device or anelectronic simulation. A fuse is a disposable thermal trip unit with noinstantaneous capability.

In addition to overload and short circuit protection, there isdeveloping interest in protection in dc electrical systems against arcfaults. Arc faults involve a highly concentrated region of heatproduction, a type of “hot spot”, that can result in insulationbreakdown, production of combustion products, and the ejection of hotmetal particles. It can also result from broken conductors or poorconnections.

Arc faults can be series or parallel. Examples of a series arc are abroken wire where the ends are close enough to cause arcing, or a poorelectrical connection. Parallel arcs occur between conductors ofdifferent potential including a conductor and ground. Arc faults occurin series with the source and series arcs are further in series with theload. Arc faults have a relatively high impedance. Thus, a series arcresults in a reduction in load current and is not detected by the normaloverload and overcurrent protection of conventional protection devices.Even the parallel arc, which almost always draws current in excess ofnormal rated current in a circuit, produces currents which can besporadic enough to yield RMS values less than that required to produce athermal trip, or at least delay operation. Effects of the arc voltageand line impedance often prevent the parallel arc from reaching currentlevels sufficient to actuate the instantaneous trip function.

For many reasons, automotive circuits will be migrating to highervoltages such as 36 or 42 volts which are disproportionately more proneto damage from arcs than the present 14 volt circuits, due principallyto the arc voltage being between 12 and 30 volts. Even 28-volt circuits,common in the aerospace industry, have been shown to provide anenvironment that supports sustained arcing. The single most aggravatingfactor beyond that found in residential power systems is vibration withsignificant humidity and dirt sometimes being aggravating factors. Inaddition, the telecommunications field uses 24 volt (and may migrate to48 volt) dc systems which are susceptible to arcing. Arcs at thesevoltages cannot preexist, i.e., must be “drawn” by a contact beingseparated. If they are initially extinguished to an open circuit, theyshould not reoccur, in theory. But the presence of carbonization or theintroduction of other contaminants dynamically, ionized gas (very shortlived) and vibration, which can recontact the surfaces, can makemultiple occurrences not uncommon. This is particularly true of a movingvehicle travelling through the elements.

The arcing, and particularly parallel arcing, in dc electrical systemscauses noise which can be exploited to detect the phenomena.Unfortunately, there are other sources of noise in the dc systems. Forinstance, dc power generated by an alternator equipped with a rectifiertypically has a ripple content. It is now becoming more common to usepulse width modulation to control loads in dc systems. Pulse widthmodulation generates steps in current that introduce spurious highfrequency signals. In addition, low energy positive and negative spikesof twice nominal voltage (and current) values can be introduced intothese dc systems by the turning off of inductive loads. Also,bidirectional currents can be introduced by regenerative breaking,battery charging, and an integrated starter/generator in automotive dcsystems.

There is a need, therefore, for an improved apparatus and method fordetecting arcing in dc electrical systems.

There is also a need for such apparatus in a method which is immune tonuisance detection in response to other noise that can be present insuch dc systems.

There is an additional need for such an apparatus and method which iseconomical to manufacture and maintain.

SUMMARY OF THE INVENTION

These needs and others are satisfied by the invention which is directedto apparatus for detecting arcing in a dc electrical system havingcyclic disturbances and comprises a sensor sensing the current in the dcsystem and generating a sensed dc current signal, a bandpass filterfiltering the sensed dc current signal with a pass band selected forsensitivity to arcing noise in the dc electrical system to generate afiltered current signal, and processing means processing the filteredcurrent signal. The processing means integrates the filtered currentsignal over repetitive intervals to generate integrated filtered currentvalues. It then generates a time attenuated accumulation of theintegrated filtered current values and produces an output signalindicating the presence of arcing when the time attenuated accumulationreaches a predetermined value. In order to be able to implement theinvention utilizing an inexpensive digital processor, which has asampling capability slower than the pass band of the bandpass.filtersensitive to arcing noise, the processing means includes a resettableanalog integrator which integrates the absolute value filtered currentsignal. The inexpensive digital processor then samples the integratedvalues during each repetitive period and uses the digitized results tocalculate the time attenuated accumulation.

In order to further immunize the apparatus from nuisance generation ofan arcing signal, only integrated values which are generated duringpredetermined time periods following instances where the sensed dccurrent signal exceeds an average value of the sensed dc current signalin a predetermined manner, such as by a selected margin or multiple, areused to generate the time attenuated accumulation. Thus, the processingmeans can include a comparator which compares the sensed dc currentsignal to the average sensed dc current signal and enables themicroprocessor to accept samples for a predetermined time period whichcan extend to multiple integration cycles.

As another aspect of the invention, the average sensed dc current signalwhich can be generated by a low pass filter can also be processed by themicroprocessor to provide overload protection.

In accordance with an additional aspect of the invention, the runningaverage of a selected number of the most recent integrated filteredcurrent values before the present value exceeds the average currentvalue in the predetermined manner is subtracted from the present valuefor use in generating the time attenuated accumulation in order tofurther desensitize the apparatus to the cyclic disturbances such asthose caused by a pulse width modulated load.

Furthermore, the invention embraces a method of detecting arcing in dcelectrical systems having cyclic disturbances which includes sensingcurrent in the dc system and generating a sensed dc current signal,bandpass filtering the sensed dc current signal in a pass band selectedfor sensitivity to arcing noise in the dc electrical system to generatean absolute value current signal, repetitively integrating the absolutevalue bandpass filtered signal over repetitive periods, generating anaverage sensed dc current signal, and processing the integrated signal,the sensed dc current signal and the average sensed dc current signal todetermine the presence of arcing.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the invention can be gained from the followingdescription of the preferred embodiments when read in conjunction withthe accompanying drawings in which:

FIG. 1 is a schematic diagram of the apparatus in accordance with theinvention.

FIG. 1A shows alternative placement of the current sensor in theapparatus of FIG. 1.

FIG. 2 is a flow diagram of an exemplary routine implemented by amicroprocessor which forms part of the apparatus of FIG. 1.

FIG. 3 illustrates a modification to a portion of the flow diagram ofFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the invention is applied to the detection ofarcing, and overloads, in a dc electrical system 1 which includes a dcbus 3 powered by a source 5, which may be, for instance, an alternatorincorporating a rectifier. A battery 7 provides alternative power to thedc bus 3 and can be charged by the source 5. Loads 9 are energized bythe dc bus 3.

The dc electrical system 1 can be an automotive system, an aerospacesystem, a telecommunications system, or other dc systems operating at avoltage susceptible to arcing, e.g., about 24 volts and above. Theinvention is directed to apparatus and a method for detecting parallelarcs in such systems. That is, arcs between conductors or betweenconductors of different potential or a conductor and ground. Such arcingproduces spurious high frequency noise. This activity must bediscriminated from other sources of noise in the dc electrical system,much of which tends to be cyclic. For instance, in automobile electricalsystems the rectified current generated by the alternator can produce acripple in the dc current. In addition, some of the loads are controlledby pulse width modulation, which can produce high frequency noise. Otherdisturbances that must be discriminated in the system include spikescaused by the turning off of inductive loads.

The arc detection apparatus 11 in accordance with the invention analyzesthe high frequency noise associated with arcing in the dc electricalsystem 1. This apparatus 11 is powered by a power supply 12 energized bythe dc bus 3 and includes a sensor 13, such as, for example, a Halleffect or a shunt device, which generates a sensed dc current signal 15.It is advantageous to locate the sensor 13 adjacent the battery 7 toincrease sensor sensitivity to high frequency noise due to the lower acimpedance of the battery relative to that of typical loads 9 and thesource 5. The sensed dc current signal 15 is applied to a bandpassfilter 17 having a pass band sensitive to arcing noise. In the exemplaryapparatus the passband has a center frequency of about 3 KHz and a Q ofabout 5. The output of the bandpass filter 17 is passed through anabsolute value circuit 19 to produce an absolute value bandpass filtersignal 21. The absolute value circuit 19 permits the apparatus torespond to positive and negative steps in the ac coupled bandpassfiltered waveform.

The apparatus 11 further includes processing circuitry 23 for processingthe absolute value bandpass filter signal 21 to detect arcing. Thisprocess circuitry 23 can be all digital. However, one of the goals ofthe invention is to provide an inexpensive apparatus. As implemented inthe exemplary embodiment, the processing circuitry 23 includes alow-cost microcomputer 25, such as a PIC microcomputer. Thismicrocomputer 25 does not have the capability of digitizing the absolutevalue bandpass filtered signal at a high enough frequency required toavoid aliasing. Accordingly, the processing circuitry 23 includes aresettable analog integrator 27, which over repetitive samplingintervals integrates the absolute value filtered current signal 21 togenerate an integrated filtered current signal 29. This analogintegrated filtered current signal 29 is digitized by A/D converter inthe microcomputer 25 for analysis by an arc fault routine, described indetail below, but which generates a time attenuated accumulation of thesamples of the integrated filtered current signal and produces an outputin the form of an arc fault signal when the time attenuated accumulationreaches a predetermined value. The microcomputer 25 resets the analogintegrator 27 after each sample is taken by providing a reset signal 31,which is 10 μsec in the exemplary system.

In order to further immunize the apparatus 11 from nuisance activation,the noise must exceed the average value of the dc current in apredetermined manner. Thus, a low pass filter 33, generates an averagesensed dc current signal 35 from the sensed dc current signal 15. Thesensed dc current signal 15 is then compared with this average sensed dccurrent signal 35 in a high speed comparator 37. By high speed, it ismeant that the comparator must have a response frequency which is atleast twice the high end of the frequency response of the bandpassfilter 17. In the exemplary apparatus, this is 10 KHz. A pulse stretcher39 maintains the logic output of the high speed comparator 37 forperiods of time which extends over one or more sampling intervals of themicroprocessor 25. In the exemplary embodiment of the invention thesampling rate of the microprocessor 25 is 1 KHz and the pulse stretcherextends the logic output of the high speed comparator 37 for about 1-5milliseconds. This output of the pulse stretcher is an enable signal 41which is input to the microprocessor 25 and enables the arc faultroutine of the microprocessor to only add those samples generated duringthe duration of the enable signal to the time attenuated accumulation.

The amount by which the sensed dc current signal 15 must exceed theaverage sensed dc current signal 35 in order for the high speedcomparator 37 to generate an enable signal 41 can be based on variouscriteria. For example, the enable signal 41 can be generated when thesensed dc current is above the worst case sum of pulse width modulatedload currents, or, for instance, when the sensed dc current signal istwo or three times the average sensed dc current signal value. Thelatter setting recognizes the fact that parallel arcs sporadically andinstantaneously greatly exceed the low frequency load current. Theoperative point is to set the threshold at a level which minimizesnuisance responses to other noise in the dc electrical system.

FIG. 1A shows a modification to the arrangement shown in FIG. 1 in whichthe sensor 13 is located in a battery cable 7 c rather than on the dcbus 3. While this alternative arrangement will not detect the steadystate dc current supplied by the source 5 to the load 9, it will stilldetect arcing currents and cyclic disturbances, such as pulse widthmodulation, anywhere in the dc electrical system 1 because of the low acimpedance of the battery. Its particular advantage is that the overloadroutine run by the microprocessor can be used to provide overloadprotection for the battery cable, both when the battery is supplying theloads, and also during battery charging and regenerative braking. Such afeature is especially useful where the battery is remotely located, suchas in the trunk of a vehicle.

FIG. 2 illustrates the exemplary routine 43 run by the microprocessor25. The sampling clock 45 establishes the one KHz clock rate whichgoverns the repetitive analog to digital conversion at 47 of theintegrated filtered current signal 29 on the analog integrator 27. Aftereach sample is taken, the integrator is reset at 49 by generation of the10 μsec reset signal 31. As was previously discussed, only the samplestaken during a predetermined time period after the sensed dc currentexceeds the average sensed dc signal in the prescribed manner are usedby the microprocessor in determining the presence of arcing. Thus, themicroprocessor implements a timer which is started by the enable signal41 from the high speed comparator 37. As indicated in FIG. 2, if thetimer is not on at 51, but the enable signal 43 is high at 53, then aone shot timer for K.t milliseconds is set at 55 where K is, forinstance, from 1 to about 5 milliseconds. If the timer is on at 51, thenthe sample is added to the time attenuated accumulation (TAA) at 57. Thetime attenuated accumulation is then decremented by K.decarc at 59.Various functions can be used to decrement the TAA such as a fixedamount per cycle or a percentage of the accumulated value. If the timeattenuated accumulation has not reached the limiting value Karc at 61,then the program returns at 63 waiting for the next sample. If however,the limit for the time attenuated accumulation has been reached at 61,an output signal indicating an arc pulse in a form of two Hz pulses isgenerated at 65. The routine 43 also takes periodic samples at 67 of theaverage sensed dc current signal 35 at the rate determined by thesampling clock 45. The samples are then scaled in accordance with theoverload curve such as that designated by SAE J1284 at 69. The scaledvalue is then added to the overload accumulator at 71. Next, overloadaccumulator is time decremented by K.decol at 73 to implement thedesired overload response curve. If the accumulated overload value hasnot reached the overload limit Kol at 75, the routine waits for the nextsample at 77. If the overload trip limit is reached at 75, then a latchis activated at 75 to produce an overload trip signal at 79.

In the above described embodiment of the invention, the output of thehigh speed comparator 37 enables the collection by the microcomputer 25of the 3 KHz activity from the resettable integrator 27. The purpose ofthis is to collect information only when the instantaneous current isgreater in a predetermined manner than the average current. Thisprevents the pulse width modulated signals from contributing to the timeattenuated accumulation if sufficiently high current spikes are notpresent.

However, the pulse width modulated signals will contribute to the totalintegration when such spikes are present and the comparator enables themicrocomputer. To remove the effect of the pulse width modulatedsignals, the microprocessor can be programmed to continuously track theaverage activity in the 3 KHz band (the signal 21) over a selected time,for example, the last five sampling cycles. The average activity can bea simple average or weighted average. Then, when the high speedcomparator 37 enables the microcomputer 25, the average 3 KHz signalrecorded at the last sample before the enable signal is generated can besubtracted from the enabled 3 KHz activity with the results used togenerate the time attenuated accumulation. FIG. 3 illustratesapplication of this feature to the portion of the routine of FIG. 2,which generates the arcing signal. As can be seen, after the integratoris reset at 49, the running average of the last five samples isgenerated at 50. If the comparator goes high at 53 to initiate the oneshot timer at 55, the running average it generated prior to detection ofthe arc is latched at 56 and is subtracted from the present value of theintegrated filtered current signal at 56 a with the difference added tothe time attenuated accumulation at 57.

As can be appreciated, the method of detecting arcing in dc electricalsystems in accordance with the invention includes sensing the dccurrent, such as by the sensor 13, bandpass filtering this sensed dccurrent signal, such as in the bandpass filter 17 and taking theabsolute value of the filtered signal, such as by the absolute value ofcircuit 19 to generate an absolute value filter current signal which isintegrated over repetitive periods of time to generate an integratedbandpass filter current signal. The method also includes generating anaverage sensed dc current signal and processing the integrated bandpassfilter signal, the sensed dc current signal and the average sensed dccurrent signal to determine the presence of arcing. More particularly,the method includes analog integration of the absolute value bandpassfiltered current signal and sampling that integrated signal periodicallywith a microprocessor which then uses the samples to generate a timeattenuated accumulation which generates an output signal indicatingarcing when a selected limit for the time attenuated accumulation isreached.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular arrangements disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the claims appended and any and all equivalents thereof.

What is claimed is:
 1. Apparatus for detecting arcing in a dc electricalsystem having cyclic disturbances, the apparatus comprising: a sensorsensing current in the dc electrical system and generating a sensed dccurrent signal; a bandpass filter filtering the sensed dc current signalin a pass band selected for sensitivity to arcing noise in the dcelectrical system to generate a filtered current signal; and processingmeans integrating the filtered current signal over repetitive intervalsto generate integrated filtered current values, generating a timeattenuated accumulation of the integrated filtered current values, andgenerating an output signal when the time attenuated accumulationreaches a predetermined value.
 2. The apparatus of claim 1, wherein theprocessing means comprises a resettable analog integrator whichrepetitively generates the integrated filtered current values over therepetitive intervals, and digital processing means processing theintegrated filtered current values to generate the time attenuatedaccumulation and the output signal when the time attenuated accumulationreaches the predetermined value.
 3. The apparatus of claim 2, whereinthe processing means further includes means generating from the senseddc current signal an average dc current signal and the digitalprocessing means includes overload means processing the average dccurrent signal and generating the output signal when the average dccurrent signal exceeds predetermined time/current limits.
 4. Theapparatus of claim 2, wherein the processing means includes enablingmeans enabling the digital processing means to add integrated filteredcurrent values to the time attenuated accumulation only forpredetermined time periods after the sensed dc current exceeds theaverage dc current in a predetermined manner.
 5. The apparatus of claim4, wherein the predetermined time periods comprise more than onerepetitive interval.
 6. The apparatus of claim 4, wherein the enablingmeans comprises a low pass filter generating the average sensed dccurrent signal and a comparator comparing the sensed dc current signalto the average sensed dc current signal and generating an enable signalenabling the digital processing means when the sensed dc current signalexceeds the average sensed dc current signal in a predetermined manner.7. The apparatus of claim 6, wherein the digital processing meansincludes overload means processing the average sensed dc current signaland generating the output signal when the average sensed dc currentsignal exceeds predetermined time/current limits.
 8. The apparatus ofclaim 7 adapted for use with a dc electrical system having a batteryconnected to a load and source by a battery cable wherein the sensorsenses current in the battery cable.
 9. The apparatus of claim 2 whereinthe digital processing means includes means generating a running averageof a selected number of most recent integrated filtered current valuesand means generating a difference between a present integrated filtercurrent signal value and the running average, which difference is usedby the means generating the time attenuated accumulation.
 10. Theapparatus of claim 1 adapted for use with a dc electrical system havinga battery connected to a load and source by a battery cable wherein thesensor senses current in the battery cable.
 11. Apparatus for detectingarcing in dc electrical systems having cyclic disturbances, theapparatus comprising: a sensor sensing current in the dc system andgenerating a sensed dc current signal; and processing means comprising:a bandpass filter filtering the sensed dc current signal in a pass bandselected for sensitivity to arcing noise in the dc electrical system togenerate an absolute value filtered current signal; a resettable analogintegrator integrating the absolute value filtered current signal overrepetitive intervals to generate an integrated filtered current signal;and a digital processor taking samples of the integrated filteredcurrent signal generated by the resettable analog integrator during therepetitive intervals, resetting the analog integrator after each sample,generating a time attenuated accumulation of the samples and generatingan output signal when the time attenuated accumulation reaches apredetermined level.
 12. The apparatus of claim 11, wherein theprocessing means further includes a low pass filter generating anaverage sensed dc current signal and wherein the digital processor hasoverload means processing the average sensed dc current signal andgenerating the output signal when the average sensed dc signal exceedspredetermined time/current limits.
 13. The apparatus of claim 12 adaptedfor use with a dc electrical system having a battery and wherein thesensor is coupled to the dc electrical system adjacent the battery. 14.The apparatus of claim 13 adapted for use with a dc electrical systemwherein the battery is connected a source and a load through a batterycable and wherein the sensor is coupled to the dc electrical system atthe battery cable.
 15. The apparatus of claim 11, wherein the bandpassfilter has a predetermined center frequency and wherein the digitalprocessor comprises a microcomputer taking samples at a rate below thecenter frequency of the bandpass filter.
 16. The apparatus of claim 15,wherein the bandpass filter has a center frequency of about 3 KHz. 17.The apparatus of claim 16, wherein the microcomputer takes samples at arate of about 1 KHz.
 18. The apparatus of claim 15, wherein theprocessing means includes a low pass filter generating an average senseddc current signal and a comparator enabling the microprocessor to takethe samples of the absolute value filtered current signal only duringpredetermined time periods after the sensed dc current signal exceedsthe average sensed dc current signal in a predetermined manner.
 19. Theapparatus of claim 18, wherein the digital processor includes meansgenerating a running average of a selected number of the most recentsamples and means generating a difference between the present sample andthe running average generated just before the sensed dc current signalexceeds the average sensed dc current signal in the predeterminedmanner, which difference is used by the digital processor in generatingthe time attenuated accumulation.
 20. The apparatus of claim 18, whereinthe comparator has a frequency response at least twice that of the passband frequency of the bandpass filter.
 21. The apparatus of claim 20,wherein the bandpass filter has a center frequency of about 3 KHz andwherein the comparator has a frequency response of at least about 10KHz.
 22. The apparatus of claim 20 adapted for use with a dc electricalsystem having a battery and wherein the sensor is coupled to the dcelectrical system adjacent the battery.
 23. A method for detectingarcing in a dc electrical system having cyclic disturbances, the methodcomprising: sensing current in the dc electrical system and generating asensed dc current signal; bandpass filtering the sensed dc currentsignal in a pass band selected for sensitivity to arcing noise in the dcelectrical system to generate an absolute value filtered current signal;generating an average sensed dc current signal from the sensed dccurrent signal; processing the absolute value filtered current signal,the average sensed dc current signal and the sensed dc current signal todetermine the presence of arcing.
 24. The method of claim 23, whereinthe step of processing comprises comparing the sensed dc current signalwith the average sensed dc current signal and digitally processingsamples of the absolute value filtered current signal taken during apredetermined time period after the sensed dc current signal exceeds theaverage sensed dc current signal in a predetermined manner to determinethe presence of arcing.
 25. The method of claim 24 wherein processingcomprises generating a running average of a selected number of mostrecent samples of the absolute value filtered current signal, and onlyusing a difference between the value of the present sample of theabsolute value filtered current signal and the running average generatedat the sample before the sensed dc current signal exceeds the average dccurrent signal to determine the presence of arcing.